Synthesis and Reactivity of Manganese Complexes Bearing Anionic PNP- and PCP-Type Pincer Ligands toward Nitrogen Fixation

A series of manganese complexes bearing an anionic pyrrole-based PNP-type pincer ligand and an anionic benzene-based PCP-type pincer ligand is synthesized and characterized. The reactivity of these complexes toward ammonia formation and silylamine formation from dinitrogen under mild conditions is evaluated to produce only stoichiometric amounts of ammonia and silylamine, probably because the manganese pincer complexes are unstable under reducing conditions.


Introduction
The production of ammonia from dinitrogen in the air, known as nitrogen fixation, has been an essential chemical process in industry and biology. The industrial nitrogen fixation, known as the Haber-Bosch process, requires harsh reaction conditions and dihydrogen derived from fossil fuels [1]. The development of new systems capable of fixing dinitrogen into ammonia efficiently under mild reaction conditions has remained a challenging goal.
Based on these backgrounds, we have focused on the development of manganesecatalyzed nitrogen fixation under mild reaction conditions. Manganese is the third most abundant transition-metal on the earth. However, successful examples of manganesecatalyzed nitrogen fixation under mild conditions are limited to the silylamine formation from dinitrogen [35,39]. Herein, we have prepared a series of manganese complexes bearing the anionic pyrrole-based PNP-type pincer ligand and the anionic benzene-based PCP-type pincer ligand to investigate their reactivity for the ammonia and silylamine formation under mild reaction conditions.

Synthesis and Characterization of Manganese Complexes
The reaction of [MnCl 2 (thf) 2 ] (thf = tetrahydrofuran) with Li-PNP (PNP = 2,5-bis(ditert-butylphosphinomethyl)pyrrolide) in the presence pyridine in THF at room temperature for 18 h gave a Mn(II) complex bearing the PNP-type pincer ligand [Mn(PNP)Cl(pyridine)] (1) in 48% yield (Scheme 1). The structure of 1 was confirmed by X-ray analysis, and an ORTEP drawing of 1 is shown in Figure 1. Table 1 presents selected bond lengths and angles of 1. Complex 1 has a distorted square pyramidal geometry, where pyridine and chloride ligands are coordinated to the cobalt center supported by the PNP ligand. Complex 1 was paramagnetic with an effective solution magnetic moment of 5.8 ± 0.5 µ B at room temperature by Evans' method, in agreement with the theoretical value of S = 5/2 state (5.9 µ B ). This result indicates that the manganese center of 1 is assigned as a d 5 high-spin state.   We attempted to prepare the corresponding manganese-dinitrogen complex by reduction of 1 with 1.1 equiv of KC 8 in THF at room temperature under 1 atm N 2 atmosphere (Scheme 2). However, no dinitrogen-coordinated species was observed by the IR spectrum. Instead, the recrystallization of the reaction mixture afforded a Mn(II) complex bearing the two PNP ligands [Mn(PNP) 2 ] in 21% yield, which was previously reported by Tonzetich and coworkers [56].  Figure 1, and bond lengths and angles of 2 are listed in Table 2. Complex 2 has a four-coordinated manganese center with a distorted tetrahedral structure. The Mn-P lengths of 2 (ca. 2.76 Å) are slightly longer than those of 1 (ca. 2.72 Å), while the P-Mn-P angle of 2 (132 • ) is smaller than that of 1 (147 • ). These structural differences between 1 and 2 may derive from the different coordination geometry around the cobalt centers. Complex 2 displayed a room-temperature magnetic moment of 6.0 ± 0.6 µ B in solution, which indicates an S = 5/2 high-spin Mn(II) state.  The reaction of [MnBr 2 (thf) 2 ] with Li-PCP (PCP = 2,6-bis(di-tert-butylphosphinomethyl) phenyl), which was generated in situ from Br-PCP with n-butyllithium, in the presence of 1,2-dibromoethane as an oxidant in THF at room temperature for 4 h gave a Mn(III) complex bearing the PCP-type ligand [Mn(PCP)Br 2 ] (3) in 44% yield (Scheme 4). X-ray analysis revealed the molecular structure of 3, and an ORTEP drawing and selected metric parameters are shown in Figure 1 and Table 3, respectively. The manganese center of 3 has a distorted square pyramidal geometry with two bromide ligands and the PCP ligand. One of the bromide ligands occupies the trans position to the C atom of the PCP ligand to make an equatorial plane, and the other is in the axial position. Shorter Mn-P bonds (ca. 2.49 Å) of 3 compared to those of 1 and 2 (2.72-2.76 Å) can be attributable to the six-membered backbone of the PCP ligand. The solution magnetic moment of 3 was 4.3 ± 0.4 µ B at room temperature. Because this value is close to the theoretical value for an S = 2 high-spin state (4.9 µ B ), 3 is assignable as an S = 2 state. Reported Mn(III)-aryl complexes exhibited effective magnetic moments between 4.8-5.3 µ B [57][58][59].

Attempted Catalytic Ammonia Formation Using 1-3 as Catalysts
We investigated the catalytic reduction of dinitrogen into ammonia or hydrazine using the synthesized manganese complexes 1-3 as catalysts under the conditions previously applied for our group [46][47][48], which were initially reported by Peters [19]. Typical results are shown in Table 4. The reaction of an atmospheric pressure of dinitrogen with 40 equiv of KC 8 as a reductant and 38 equiv of [H(OEt 2 ) 2 ]BAr F 4 (Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) as a proton source in the presence of a catalytic amount of 1 in Et 2 O was conducted at −78 • C for 1 h. As a result, neither ammonia nor hydrazine was obtained, while 7.8 equiv of dihydrogen based on the Mn atom were formed as a product (Table 4, entry 1). The use of 2 as a catalyst afforded 1.4 equiv of ammonia and 7.5 equiv of dihydrogen under the same reaction conditions (Table 4, entry 2), where 1 equiv of ammonia is considered to be derived from the N(SiMe 3 ) 2 ligand in 2. We also attempted catalytic ammonia formation using complex 3 as a catalyst; however, no formation of ammonia nor hydrazine was observed. (Table 4, entry 3). These results indicate that the newly synthesized manganese complexes bearing the anionic PNP-type or PCP-type ligands did not work as catalysts for catalytic ammonia or hydrazine formation from dinitrogen.

Attempted Catalytic Silylamine Formation Using 1-3 as Catalysts
We then tested 1-3 for their potential as catalysts for the formation of silylamine (N(SiMe 3 ) 3 ), which is easily converted to ammonia upon hydrolysis. Hence silylamine formation is regarded as an alternative nitrogen fixation process [8]. Typical results are shown in Table 5. The reaction of an atmospheric pressure of dinitrogen with 600 equiv of Na as a reductant and 600 equiv of Me 3 SiCl as a silylating reagent in the presence of 1 as a catalyst in THF was performed at room temperature for 20 h, followed by hydrolysis of the reaction mixture to convert silylamine to ammonia for quantification. This reaction afforded only 0.4 equiv of ammonia based on the Mn atom in the catalyst ( Table 5, entry 1). When KC 8 was used as a reductant in place of Na, 1.4 equiv of ammonia were obtained based on the Mn atom in the catalyst (Table 5, entry 2). The catalytic activity of 2 toward silylamine formation was also investigated in the same reaction conditions to give less than 2 equiv of ammonia (Table 5, entries 3 and 4). The use of 3 as a catalyst in the presence of Na or KC 8 as a reductant afforded 2.5 or 2.2 equiv of ammonia based on the Mn atom in the catalyst, respectively, which are regarded as superstoichiometric amounts (Table 5, entries 5 and 6). These results suggest that these manganese complexes did not work as catalysts for silylamine formation under the current conditions.

Materials and Methods
3.1. General Methods 1 H NMR (400 MHz), 13 C{ 1 H} NMR (100 MHz), and 31 P{ 1 H} NMR (162 MHz) spectra were recorded on a JEOL ECS-400 spectrometer (Tokyo, Japan) or a JEOL ECZ-400S spectrometer in a suitable solvent, and spectra were referenced to residual solvent ( 1 H, 13 C{ 1 H}) or external standard ( 31 P{ 1 H}: H 3 PO 4 ). IR spectra were recorded on a JASCO FT/IR 4100 Fourier Transform infrared spectrometer (JASCO, Tokyo, Japan) or a Shimadzu IRSpirit spectrometer (Shimadzu, Kyoto, Japan). UV-vis absorption spectra were recorded on a Shimadzu UV-1850. Magnetic susceptibility was measured in C 6 D 6 using the Evans method. Elemental analyses were performed at the Microanalytical Center of The University of Tokyo.
All manipulations were carried out under an atmosphere of nitrogen or argon by using standard Schlenk techniques or glovebox techniques, unless otherwise stated. Solvents were dried by general methods and degassed before use. Me 3 SiCl was distilled prior to use. Li-PNP [47], H-PNP [60][61][62] [20,66], and KC 8 [67] were prepared according to the literature methods. All the other reagents were commercially available (reagent grade from TCI (Tokyo, Japan), FUJIFILM Wako Pure Chemical (Osaka, Japan), and Sigma-Aldrich (St. Louis, MO, USA)) and used as received.

Preparation of PCP-Br
A mixture of 2-bromo-1,3-bis(bromomethyl)benzene (2.61 g, 7.61 mmol) and t Bu 2 PH (2.94 g, 20.1 mmol) in acetone (55 mL) was stirred at reflux temperature for 2 h. The mixture was cooled to room temperature, and then the solvent was removed in vacuo. The residue was washed with Et 2 O (8 mL, 3 times). NaOAc (5.0 g), Et 2 O (20 mL), and water (15 mL) were added to the white residue. The product was extracted by Et 2 O (15 mL, 3 times). The combined extracts were dried over anhydrous MgSO 4 , and then the mixture was filtered. The filtrate was evaporated to dryness to afford Br-PCP as a white solid (2.83 g, 5.98 mmol, 79%). 1

Preparation of 1
To a solution of Li-PNP (195 mg, 0.501 mmol) in THF (10 mL), pyridine (82 µL, 1.02 mmol) and [MnCl 2 (thf) 2 ] (134 mg, 0.496 mmol) were added at room temperature. The reaction mixture was stirred at room temperature for 18 h, and the solvent was removed in vacuo. Et 2 O (10 mL) was added to the brown residue. The suspension was filtered through Celite, and the filter cake was washed with Et 2 O (3 mL, 4 times). The combined filtrate was concentrated into 5 mL. The resultant mixture was kept at −30 • C to give a yellow solid, which was collected by filtration, washed with a small amount of cold pentane, and dried in vacuo to afford 1 as a yellow solid (132 mg, 0.239 mmol, 48%). Single crystals of 1 suitable for X-ray crystallography were obtained as yellow crystals from Et 2 O at −30 • C. Magnetic susceptibility: µ eff = 5.8 ± 0.5 µ B in C 6 D 6 at 298 K. Anal. Calcd. for C 27

Preparation of 2
To a solution of [Mn{N(SiMe 3 ) 2 }](thf) 2 (97.7 mg, 0.188 mmol) in THF (2 mL), a solution of H-PNP (68.3 mg, 0.178 mmol) in THF (2 mL) was added dropwise at room temperature. The reaction mixture was stirred at room temperature for 14 h, and then the solvent was removed in vacuo. Pentane (2 mL) was added to the pale-yellow residue. The suspension was filtered through Celite, and the filter cake was washed with pentane. The combined filtrate was concentrated into 1 mL. The resultant mixture was kept at −30 • C to give a pale orange solid, which was collected by decantation, washed with a small amount of cold pentane, and dried in vacuo to afford 2 as a pale orange solid (80.0 mg, 0.134 mmol, 75%). Single crystals of 2 suitable for X-ray crystallography were obtained as yellow crystals from pentane at −30 • C. Magnetic susceptibility: µ eff = 6.0 ± 0.6 µ B in C 6 D 6 at 298 K. Anal. Calcd. for C 28

Attempted Catalytic Ammonia Formation Using 1-3 as Catalysts
A typical experimental procedure for reduction of dinitrogen to ammonia or hydrazine using 1 is described below. In a 50 mL Schlenk flask, 1 (5.2 mg, 10.0 µmol), KC 8 (54.1 mg, 0.400 mmol) and [H(OEt 2 ) 2 ]BAr F 4 (385 mg, 0.380 mmol) were placed. After the mixture was cooled to −78 • C, cold Et 2 O (5 mL) was added to the mixture. After the mixture was stirred at −78 • C for 1 h, the mixture was warmed to room temperature and further stirred at room temperature for 20 min. The amount of dihydrogen evolved in the reaction was determined by gas chromatography (GC) analyses. The reaction mixture was evaporated under reduced pressure, and the distillate was trapped in a dilute H 2 SO 4 solution (0.5 M, 10 mL). Aqueous solution of potassium hydroxide (30 wt%, 5 mL) was added to the residue, and the mixture was distilled into the same dilute H 2 SO 4 solution (0.5 M, 10 mL). The amount of NH 3 present in H 2 SO 4 solution was determined by the indophenol method [68]. The amount of NH 2 NH 2 present in H 2 SO 4 solution was determined by the p-(dimethylamino)benzaldehyde method [69].

Attempted Catalytic Silylamine Formation Using 1-3 as Catalysts
A typical experimental procedure for reduction of dinitrogen to silylamine using 1 is described below. In a 50 mL Schlenk flask, 1 (2.4 mg, 5.0 µmol) and KC 8 (406 mg, 3.00 mmol) were placed. After Et 2 O (6 mL) and Me 3 SiCl (380 µL, 3.00 mmol) were added to the mixture in the Schlenk, the mixture was stirred at room temperature for 40 h under N 2 (1 atm). Dilute H 2 SO 4 solution (0.5 M, 10 mL) was added to the mixture. The mixture was stirred at room temperature for 1 h. Aqueous solution of KOH (30 wt%, 5 mL) was added to the reaction mixture, and the mixture was distilled into another dilute H 2 SO 4 solution (0.5 M, 10 mL). The amount of ammonia was determined by the indophenol method [68].

X-ray Diffraction
Crystallographic data of 1-3 are summarized in Table 6. Diffraction data for 1-3 were collected for the 2θ range of 4 • to 55 • at −100 • C (for 1 and 2) or −180 • C (for 3) on a Rigaku R-AXIS RAPID imaging plate area detector (Rigaku, Tokyo, Japan) with multi-layer mirror monochromated Mo-Kα (λ = 0.71075 Å) radiation with VariMax optics. Intensity data were corrected for Lorentz and polarization effects and for empirical absorptions (ABSCOR [70] for 1-3), while structure solutions and refinements were performed by using the CrystalStructure package [71]. The positions of non-hydrogen atoms were determined by direct methods (SHELXT version 2014/5 [72] for 1 and 3; SHELXS version 2013/1 [73] for 2) and subsequent Fourier syntheses (SHELXL [74] version 2016/6) and were refined on F o 2 using all unique reflections by full-matrix least-squares with anisotropic thermal parameters. All the hydrogen atoms were placed at the calculated positions with fixed isotropic parameters. Crystal of 1 and 3 contains an inverted structure as a disorder, which was solved as merohedral twins. Crystal of 2 contains heavy disorders among a tert-butyl group, which were solved as disorders in a ratio of 0.52:0.48 for C(21A)-C(22A) and C(21B)-C(22B). All the hydrogen atoms attached to these carbon atoms could not be located, causing checkCIF/PLATON report Alert level B (PLAT043_ALERT_1_B Calculated and Reported Mol. Weight Differ by 6.06).

Conclusions
In the current work, three manganese complexes bearing anionic PNP-and PCPtype pincer ligands were synthesized and fully characterized. Their catalytic activities for nitrogen fixation, such as ammonia and silylamine formation, were extensively studied. As a result, unfortunately, these manganese complexes did not promote the catalytic formation of ammonia or silylamine from nitrogen gas under mild reaction conditions. These results are in contrast to the previous results that vanadium, iron, cobalt, rhodium, and iridium complexes bearing the identical anionic pincer ligands worked as catalysts for ammonia formation or silylamine formation under mild reaction conditions [46][47][48]54,55]. Currently, we consider that the manganese pincer complexes are unstable under reducing conditions to cause catalyst decomposition. We hope the present result will help researchers to design nitrogen fixation systems using transition-metal complexes as catalysts.

Funding:
The present project is supported by CREST, JST (Grant JPMJCR1541). We acknowledge Grants-in-Aid for Scientific Research (Grants JP20H05671, JP20K21203, and JP20K15295) from JSPS and MEXT.

Data Availability Statement:
The crystallographic data are available from the Cambridge Crystallographic Data Centre (CCDC).